1. Field of the Invention
This invention relates generally to the digital data processing art and more particularly to a new method and apparatus for encoding binary digital data which is particularly suitable to be used in digital data communication systems and digital data storage and retrieval systems.
2. Description of the Prior Art
Data storage capacity of digital data storage and retrieval systems has been gradually increasing, and some improved methods for encoding binary digital data have contributed to the increase of storage data capacity to a large extent. Generally, the method for encoding binary data is required to have the following features:
(i) a large value of T.sub.min ;
(ii) a small value of T.sub.max ; and
(iii) is DC free,
where T.sub.min and T.sub.max represent respectively the minimum and maximum distances between encoded signal transitions. The feature (i) means that the high frequency spectra of the encoded digital signals is reduced. With this feature, the upper limit of the transmission band can be lowered, or storage capacity of the data storage system can be increased. The feature (ii) means that self clocking is possible. With this feature, the timing shift between the clock and signals is reduced, so that the storage capacity of the data storage system can be increased. As a method which is excellent in the features (i) and (ii), the so-called "3PM" method is already known as disclosed in U.S. Pat. No. 4,323,931. This method separates input binary data into successive data words each containing 3 data bits and converts each data word to a channel word containing 6 data bits. The 3PM method has such excellent features of T.sub.min =1.5T and T.sub.max =6T, but does not have the feature (iii).
In the case of retrieving data from a magnetic storage medium by means of a ring head in which differential signals can be detected, it is possible to detect peak levels of the retrieved signals, so that it is not necessary to be DC free. However, when the retrieved signals contain a large amount of noise, the differential signals must be integrated prior to being wave-formed. In this case, a DC free encoding method is desirable. Also, in the case of using a thin-film head having a magnetoresistance effect in which the differential signals are not retrieved, a DC free encoding method is desired. Moreover, in optical data storage and retrieval systems which require accurate tracking on data tracks, encoded digital signals will be added as noise to the tracking control signal if the encoding method is not DC free.
As a method having all of the above features (i), (ii) and (iii), the so-called "EFM" method is already known which is applied to the "compact disk" (trademark of Philips Corp. and Sony Corp.). This method realizes a very small value of digital sum variations (DSV, hereafter), where DSV is generally defined as a value obtained by summing data bits of encoded binary data assuming bit `1` as -1 and bit `0` as +1. The EFM method separate input binary data into data words each containing 8 data bits, converts each data word to a channel word containing 14 data bits, inserts 3 merging bits between two channel words, and converts the obtained data to NRZI code data. The merging bits are selected, by referring to the previous DSV and the following channel word, so as to reduce the total DSV, so as to thereby make the encoded data DC free. In this method, however, a complicated decoding circuit must be provided for decoding the 14-bit channel words. If the number of data bits of the channel word is reduced for simplifying the decoding circuit, then the data storage capacity is decreases due to the overhead of the 3 merging bits.